This invention relates to warning systems for what are known in the U.S. as railroad grade crossings (sometimes referred to in the U.K. as level crossings), which are locations at which railroad tracks intersect roads. The warning systems typically include lights, bells and a gate arm that blocks the road when a train is approaching the crossing.
The lights, bells and gate arms of the warning system are typically controlled by a controller that includes an input from a grade crossing predictor circuit. Grade crossing predictor circuits are able to determine a distance from the train to the crossing and can determine whether the train is approaching the crossing or moving away from the crossing. This ability allows the controller to activate the warning system with a constant warning time prior to the train reaching the crossing. These circuits typically employ tuned shunts at either end of an approach area to a crossing and work by transmitting a signal (typically a low frequency signal in the audio range) through the rails and shunts and sensing an inductance (or impedance) of the circuit formed by the track rails and shunts. When a train approaches a crossing, the train's axles and wheels create a short circuit between the rails, which lowers the total apparent inductance. By monitoring the inductance or impedance changes, the distance and speed of the train can be determined so that the warning system can be activated with a constant warning time.
Unfortunately, grade crossing predictor circuits do not work in all circumstances. For example, grade crossing predictor circuits may not work, or may not work reliably, in electrified territory (i.e., territory in which propulsion current is supplied through a third rail or a catenary wire system) and other areas with significant electromagnetic interference. In such circumstances, it is necessary to provide an alternative technique for controlling a warning system for a grade crossing.
One possible alternative technique is to control the grade crossing using a signal from a track occupancy circuit. A track occupancy circuit is a type of circuit that detects the presence or absence of a train in a section of track. A grade warning system controller can be configured such that the warning system activates when the track occupancy circuit indicates that a train is present in a section of track prior (with respect to a direction in which the train in traveling) to the grade crossing. In systems such as these, the length of the section of track monitored by the track occupancy circuit is chosen such that detection of a train traveling at the fastest authorized train speed in the block of track monitored by the track circuit will result in activation of the warning system at a desired amount of time prior to arrival of the train at the crossing.
Systems such as these suffer from a significant drawback: since the warning system activates as soon as a train is detected in the section of track monitored by the block occupancy circuit, a train traveling at a speed significantly lower than a maximum authorized speed will result in activation of the warning system a significantly longer period of time prior to arrival of the train at the crossing than is necessary or desired.